Method of processing optical fiber

ABSTRACT

There is provided a method of processing an optical fiber having a core and a clad. The optical fiber has a first facet and a second facet. The method includes fixing the optical fiber in a state in which at least a portion thereof is bent in a predetermined curvature radius, applying a resist to a region on the first facet at least including an entirety of the core, irradiating the second facet with light of a predetermined wavelength so that only the resist applied to the core in the first facet is exposed to the light through an inside of the optical fiber, developing the resist, and forming a level gap at a boundary between the core and the clad in the first facet utilizing the resist remaining after the irradiating and the developing.

Incorporation By Reference

This application claims priority of Japanese Patent Application No.2004-249806, filed on Aug. 30, 2004, the entire subject matter of theapplication being incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method of processing an optical fiberemployed in an optical communication apparatus.

An optical communication apparatus includes optical components such as alaser diode (LD), a lens that converges light from the LD, and anoptical fiber, for transmitting light emitted by the LD and modulatedaccording to information, to the optical fiber. An optical communicationmodule that serves as an ONU (Optical Network Unit), through which theoptical fiber communication is introduced into a subscriber's house,further includes a photoreceptor and a WDM (Wavelength DivisionMultiplex) filter that separates light of different wavelengths, forperforming interactive communication in which a single optical fiber isused for both transmission and reception in common.

In such an optical communication module, signal light from the LD has tobe introduced to a generally central portion of a core of the opticalfiber, so as to transmit or receive the signal light through the opticalfiber. In other words, the LD has to be precisely positioned withrespect to the core of only a few microns in diameter, of the opticalfiber. In a conventional positioning method, an amount of light emittedfrom the LD is detected, and the light from the LD is decided to beincident upon a generally central portion of the core if the lightamount satisfies a predetermined level. Normally, those opticalcomponents are firmly fixed by welding or with an adhesive, after thepositioning process.

According to the conventional positioning method, however, it isimpossible to decide how much and in which direction the incidentposition of the light from the LD is shifted, when the amount of theemitted light is below the predetermined level. Accordingly, therelative positioning between the incident position and the core of theoptical fiber has to be repeated on a “trial and error” basis, until theamount of the light emitted by the LD reaches the predetermined level,which is troublesome and time consuming.

Further, though the relative positions of the components are fixed withan adhesive upon completing the positioning operation by the abovemethod, to thereby constitute an optical communication module, thefollowing issues still remain unsettled. Firstly, when the opticalcommunication module is fabricated as above, the evaluation of theproduct cannot be executed until the adhesive completely dries after theadhesion, since deformation of or damage to the components due toshrinkage of the adhesive or the process may occur. It is thereforedifficult to achieve a high yield with such an optical communicationmodule. Secondly, if the performance level of the optical communicationmodule deteriorates with time, it is no longer possible to readjust theperformance, and hence the high-precision positioning performance cannotbe maintained.

Desirable remedies for solving the foregoing problems include actuallydetecting the incident position of the light from the LD on a lightreceiving facet of the optical fiber, so as to adjust the position suchthat the incident position coincides with the center of the core, andconstituting the optical communication module so as to constantlyperform the positioning operation with respect to the light from the LD.In order to practically carry out such remedies, the light receivingfacet of the optical fiber has to be processed so as to enable detectingthe incident position on the light receiving facet with high precision,and processing the light receiving facet so that the boundary betweenthe core and the clad can be clearly identified on the light receivingfacet of the optical fiber.

An example of processing a light receiving facet of an optical fiber isdisclosed in each of Japanese Patent Provisional Publications No.H05-107428 (hereafter, referred to as a document 1) and No. 2001-305382(hereafter, referred to as a document 2).

The documents 1 and 2 both aim at improvement in propagation efficiencyof light when optically connecting an optical fiber with another opticaldevice such as an optical waveguide. These documents disclose a methodof forming a core facet of the optical fiber in a protruding shape, orproviding a protruding member close to the core facet. The method ofprocessing disclosed in the documents 1 and 2 can be appropriatelyemployed when processing an emitting facet situated opposite to anotheroptical device, i.e. a facet serving for optical connection with anotheroptical device.

The method of forming the core facet in a protruding shape according tothe document 1 utilizes a difference in etching rate due to a differencein composition between the core and the clad, and is hence unable toprecisely process the core facet so that the core can be clearlydistinguished from the clad. Accordingly, employing an optical fiberprocessed as above does not lead to high-precision detection of anincident position of light from the LD.

In the method of forming a protruding member close to the core accordingto the document 2, it is difficult to achieve a correct alignmentbetween the optical fiber and a mask used for exposing only apredetermined portion of the optical fiber. Therefore, the methoddisclosed in the document 2 is unable to precisely process the corefacet so that the core can be clearly distinguished from the clad.Consequently, the method of processing disclosed in the documents 1 and2 cannot be employed as a method of processing a light receiving facetintended for the high-precision position detection and positioningoperation for the light receiving facet.

SUMMARY OF THE INVENTION

The present invention is advantageous in that it provides a method ofprocessing an optical fiber that enables clear distinction of a boundarybetween a core and a clad on a light receiving facet, so as to besuitably implemented in an optical communication apparatus capable ofperforming a high-precision positioning operation with respect to lightfrom an LD.

According to an aspect of the invention, there is provided a method ofprocessing an optical fiber having a core and a clad. The optical fiberhas a first facet and a second facet. The method includes fixing theoptical fiber in a state in which at least a portion thereof is bent ina predetermined curvature radius, applying a resist to a region on thefirst facet at least including an entirety of the core, irradiating thesecond facet with light of a predetermined wavelength so that only theresist applied to the core in the first facet is exposed to the lightthrough an inside of the optical fiber, developing the resist, andforming a level gap at a boundary between the core and the clad in thefirst facet utilizing the resist remaining after the irradiating and thedeveloping.

By the method thus arranged, since the optical fiber is bent in apredetermined curvature radius, a portion of the light incident upon theclad, out of the light passing through the optical fiber from the secondfacet, is attenuated a plurality of times and hence can barely affectthe exposure, even though such portion of the light reaches the firstfacet. In contrast, the light that has entered the core repeats totalreflection inside the core, to be thereby introduced only to the resistapplied to the core in the first facet. As a result, the method allowsexposing only the resist applied to the core, with high precision.

Accordingly, the method allows clearly identifying a boundary betweenthe core and the clad on the first facet, i.e. evidently exhibiting adifference in optical performance between the core and the clad on thefirst facet. Employing the optical fiber thus processed enablesdetecting an incident position of the light from a light source on thefirst facet, based on the difference in optical performance. Based onsuch detection result, a negative feedback control can be performed soas to set the incident position of the light from the light source onthe center of the core. If the optical fiber is incorporated in anoptical communication apparatus such that the first facet serves as areceiving facet for the light from the light source, the opticalcommunication apparatus can perform the positioning operation eitherconstantly or at a desired timing, and can thereby maintain the highperformance level, free from variation in environment of use orfluctuation with time.

Optionally, the method may include finishing at least a portion of anouter surface of the clad in a rough surface.

With this configuration, the attenuation effect due to scattering at theouter wall of the clad is enhanced.

Still optionally, the method may include providing at least one coatingon an outer surface of the clad.

With this configuration, the attenuation effect due to scattering at theinterface between the clad and the coating can be further enhanced.

Still optionally, the method may include providing two or more coatingson an outer surface of the clad, and finishing at least one ofinterfaces between the coatings in a rough surface.

With this configuration, the attenuation effect due to scattering at theinterface between the coatings can be enhanced.

Still optionally, the method may include providing two or more coatingson an outer surface of the clad. In this case, UV light may be used asthe light of the predetermined wavelength in the irradiating, and atleast one of the two or more coatings may be formed of a UV-absorbingmaterial.

Still optionally, a coating which is one of the two or more coatings andis formed of the UV-absorbing material may have a higher refractiveindex for the UV light than an inner adjacent coating of the coatingformed of the UV-absorbing material.

In a particular case, nylon which is opaque to the UV light may be usedas the UV-absorbing material.

Optionally, a following condition may be satisfied:20≦R≦200  (1)

-   -   where R (mm) represents the predetermined curvature radius.

In a particular case, the resist may be applied to an entire region ofthe first facet in the applying.

In a particular case, the resist may be a negative resist.

In a particular case, the resist may be a positive resist.

Optionally, the forming of the level gap may include performing asurface treatment at least within a region close to the core on the cladin the first facet so as to generate a difference in reflectance betweenthe region and the core, and stripping the resist remaining on the firstfacet subjected to the surface treatment.

In a particular case, the surface treatment may be a treatment ofvapor-depositing a metal material in a form of a thin film at leastwithin the region close to the core on the clad in the first facet.

Optionally, the forming of the level gap may include performing anetching on a region where the resist is no longer present on the firstfacet, and stripping the resist remaining on the first facet, after theetching.

Still optionally, the forming of the level gap may include filling aregion where the resist is no longer present in the first facet with amaterial that has generally the same refractive index as the opticalfiber, and stripping the resist remaining on the first facet after thefilling.

BRIEF DECRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an optical fiberbefore being processed by a method of processing according toembodiments of the present invention;

FIGS. 2A to 2F are schematic side views for explaining the method ofprocessing an optical fiber according to a first embodiment;

FIG. 3 is a schematic side view showing an optical fiber ready forexposure after a coating process and a fixing process;

FIG. 4 is a perspective view showing the optical fiber processed by themethod of processing according to the first embodiment;

FIG. 5 is a perspective view showing another optical fiber processed bythe method of processing according to the first embodiment;

FIG. 6 is a perspective view showing an optical fiber processed by amethod of processing according to a second embodiment;

FIGS. 7A to 7F are schematic side views for explaining the method ofprocessing an optical fiber according to the second embodiment;

FIG. 8 is a perspective view showing an optical fiber processed by amethod of processing according to a third embodiment;

FIGS. 9A to 9F are schematic side views for explaining the method ofprocessing an optical fiber according to the third embodiment;

FIGS. 10A to 10F are schematic side views for explaining a method ofprocessing an optical fiber according to a fourth embodiment; and

FIG. 11 is a schematic diagram showing a configuration of an opticalcommunication module including the optical fiber processed by the methodof processing according to the first embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereafter, embodiments according to the present invention will bedescribed. As described below, a method of processing an optical fiberis described in the embodiments. The optical fibers 3A, 3B, 3C, 3Dprocessed by the method according to a first embodiment, and to a secondto a fourth embodiments to be subsequently described, are all intendedfor use in an optical communication module, so as to serve to transmit asignal light from an LD (laser diode). The essence of the method ofprocessing an optical fiber according to the embodiment lies in forminga level gap of a predetermined dimension with high precision, forclearly defining optical performance of a core and a clad on a lightreceiving facet of the optical fiber to be processed. Accordingly, itbecomes possible to constantly perform the positioning of the laserdiode with respect to the optical fiber with high precision, based onthe difference in optical performance defined by the level gap in theoptical communication module implemented with the optical fiberprocessed by the method according to the embodiment.

FIG. 1 is a schematic cross-sectional view showing an optical fiber 3A(3B, 3C, or 3D) before being processed by the method of processingaccording to the respective embodiments. As shown in FIG. 1, the opticalfiber 3A (3B, 3C, or 3D) includes a main body constituted of a clad 32and a core 33. The optical fiber 3A (3B, 3C, or 3D) includes a firstfacet 31, and a second facet 34 on the opposite end to the first facet31. The main body is coated with a film 35 made of a UV-curing resin,for protection. The UV-curing resin film 35 is constituted of materialhaving a higher refractive index with respect to UV light, than the clad32. It is to be noted that the term “refractive index” herein referredto stand for a refractive index with respect to UV light, unlessotherwise stated. The UV-curing resin film 35 has an outer face, i.e. asurface processed in advance in a rough surface. The surface processingmethods include grinding. As already stated, since the UV-curing resinfilm 35 having a higher refractive index than the clad 32 is providedover the clad 32, the UV light is not totally reflected at the interfacebetween the clad 32 and the UV-curing resin film 35.

In all the embodiments described below, the method of processing isstarted with coating a surface of the UV-curing resin film 35 of theoptical fiber 3A (3B, 3C, or 3D) with material having a higherUV-absorbance than the UV-curing resin film 35 (a coating process). Inthe embodiments, nylon 36 which does not transmit the UV light isemployed as the coating material, because the UV light is used in theprocessing. The nylon 36 has a higher refractive index than theoutermost layer of the optical fiber, i.e. the UV-curing resin film 35provided for protecting the optical fiber. Accordingly, in the opticalfiber 3A to 3D, the refractive index becomes greater in a layer closerto the ambience (ambient air), i.e. in the sequence of the clad 32, theUV-curing resin film 35, and the nylon 36. Alternatively, the clad 32,the UV-curing resin film 35 and the nylon 36 may have generally the samerefractive index.

Although the first facet 31, and end portions of the UV-curing resinfilm 35 and the nylon 36 on the side of the first facet 31 areillustrated as stepped in FIG. 1, it is only for explicitness indescription. In the method of processing according to the presentinvention, practically the end portions of the UV-curing resin film 35and the nylon 36 on the side of the first facet 31 are disposed to beflush with the first facet 31. This is also the case with FIGS. 3 and 4to be subsequently described.

Upon finishing the coating with the nylon 36, the optical fiber 3A (3B,3C, or 3D) is bent at a portion in a predetermined curvature radius R,and fixed as it is (fixing process). The curvature radius R (mm) isdetermined to satisfy a condition (1).20≦R≦200  (1)

The above range protects the optical fiber from an undue load that maydamage the optical fiber, and also facilitates exposing the first facetof the optical fiber with high precision. When the curvature radiusexceeds the upper limit, the scattering effect for the UV light incidentupon the clad is lowered, in the exposing process to be described later.In contrast when the curvature radius is below the lower limit, theoptical fiber is subjected to an excessive load, by which the opticalfiber may be broken or disconnected.

The lower limit may be raised, for example up to 50 mm, depending on thestructure or material of the optical fiber 3A (3B, 3C, or 3D). In theembodiments, the curvature radius R is set at approx. 100 mm.

Once the coating process and the fixing process have been completed, themethod of processing according to the respective embodiments differsfrom one another. Hereunder, the method of processing of each embodimentwill be described by turns.

First Embodiment

FIGS. 2A to 2F are schematic side views for explaining the method ofprocessing an optical fiber according to the first embodiment. FIG. 2Adepicts the optical fiber 3A that has undergone the coating process andthe fixing process. On the optical fiber 3A bent in a predeterminedcurvature radius R and thus fixed, a resist r is applied to an entireregion of the first facet 31, in a generally uniform thickness, as shownin FIG. 2B (a resist coating process). For applying the resist in agenerally uniform thickness, a technique such as a spin coating or spraycoating may be employed. The resist r employed in the first embodimentis of a negative type.

After the resist r has been applied to the entire region of the firstfacet 31 in a uniform thickness, an exposing/developing process iscarried out. In the exposing/developing process, firstly UV light isirradiated from the second facet 34 as shown in FIG. 2C. The UV lightincident upon the second facet 34 passes through inside the core 33, tothereby reach the resist r. Irradiating thus the UV light from the sideof the second facet 34, instead of from the side of the first facet 31to which the resist has been applied, allows skipping a process ofgenerating a mask for the first facet 31, thereby achieving a simple andquick processing method.

FIG. 3 is a schematic side view showing the optical fiber 3A in theexposure process after the optical fiber 34 has undergone the coatingand fixing processes. In FIG. 3, a symbol L1 designates a light beamgenerally perpendicularly incident upon the second facet 34 and passingthrough inside the core 33, L2 designates a light beam generallyperpendicularly incident upon the second facet 34 and passing throughinside the clad 32, and L3 designates a light beam obliquely incidentupon the clad portion on the second facet 34, respectively.

As shown in FIG. 3, during the exposing process in the method ofprocessing according to the first embodiment, the light beam L1 proceedsinside the core 33 repeating total reflections substantially withoutbeing affected by the bent shape of the optical fiber, because the core33 is as fine as several microns in diameter. Thus the light beam L1 isincident upon the first facet 31 without being attenuated. In contrast,the light beams L2, L3 reach the first facet 31 after repeatingreflections at the interface between the UV-curing resin film 35 and thenylon 36, since the optical fiber 3A is bent in the preceding fixingprocess. In FIG. 3, the incident positions of the light beams L2, L3 atthe interface are indicated by symbols “P”.

On the optical fiber 3A the nylon 36 has a higher refractive index thanthe UV-curing resin film 35, as already stated. Accordingly, when thelight beams L2, L3 are reflected at the points P, the angle of the lightbeam with respect to the nylon 36 exceeds the critical angle of thetotal reflection, and thus a portion of the light beam leaks outward.More specifically, the light beams L2 and L3 are subjected to an opticalattenuation at the points P, such as scattering, transmission orabsorption into the nylon 36. This is what is called an opticalradiation loss due to bending. In addition, since the UV-curing resinfilm 35 is processed to have a rough surface, the light can be moreeffectively attenuated.

In this way, unlike the light beam L1, the light beams L2 and L3 reachthe first facet 31 after being attenuated through a plurality ofreflections at the interface between the UV-curing resin film 35 and thenylon 36. Accordingly, the light beams L2 and L3 are far less intensethan the light beam L1, upon reaching the first facet 31. Therefore,even though the light beams L2 and L3 are incident upon the resist rapplied to the clad 32, it barely contributes for the exposure of theresist r. In contrast, the light beam L1 incident upon the first facet31 through the core 33 is scarcely attenuated, and hence exposes theresist r applied to the core 33 with sufficient intensity.

The irradiation time of the UV light is determined such that the resistr applied to the core 33 can be sufficiently exposed. After theexposure, the development is performed, so that the unexposed portion ofthe resist r, i.e. the resist r applied to the clad 32, is dissolved andremoved.

FIG. 2D depicts the state of the optical fiber 3A after the developingprocess. In view of FIG. 2D, it is apparent that irradiating the UVlight from the side of the second facet 34 for exposure results inleaving only the portion of the resist r in the region corresponding tothe core 33 generally in a column shape including the core 33 as itsbottom face.

Then, a metal material m is vapor-deposited in a uniform thickness onthe first facet 31 that has just undergone the developing process, thusto form a mirror surface (vapor deposition process). FIG. 2E depicts thestate of the optical fiber 3A where the metal material m (indicated byoblique lines in FIG. 2E) has been vapor-deposited on the first facet31. Examples of the metal material m include high-reflectance metalssuch as Cr, Cu, Au and Al. When depositing the metal material m on thefirst facet 31, a sputtering process or CVD (Chemical Vapor Deposition)may be performed, instead of the vapor deposition. By thus depositingthe metal material m in a form of a thin film, a portion of the firstfacet 31 corresponding the clad 32 becomes a mirror-finished surface.After vapor-depositing the metal material m on the first facet 31, theresist r remaining on the first facet 31 is lifted off together with themetal material m vapor-deposited on the resist r, so that the core 33 isexposed (a resist stripping process). For stripping the remaining resistr, a solution of acetone or the like may be employed. FIG. 2F depictsthe optical fiber 3A from which the resist r has been stripped. At thisstage, the optical fiber 3A obtains the structure shown in FIG. 4.

Referring to FIG. 4, the first facet 31 of the optical fiber 3A has beenprocessed such that a region except the core 33, i.e. a generally entireregion of the clad 32 has a different reflectance from that of the core33. To be more detailed, by the method of processing according to thefirst embodiment, since the metal material m is coated in a form of athin film generally all over the clad 32, a level gap is provided in aform of a recessed portion on the core 33, on the first facet 31.

The optical fiber 3A shown in FIG. 4 provides a higher reflectance ofthe first facet 31, than a conventional optical fiber in which notreatment is applied to the clad 32 on the first facet 31. Accordingly,implementing the optical fiber 3A in an optical communication modulesuch that the first facet 31 serves as the light receiving facet for thelight from the laser diode allows detecting an amount of light incidentupon the clad 32 and then reflected thereby. This leads tohigh-precision adjustment of the incident position to the center of thecore 33, on the light receiving facet (first facet 31).

The foregoing is the method of processing an optical fiber according tothe first embodiment. In this method, the mirror surface is formed allover the first facet 31. However, the mirror surface may be formed onlyin the vicinity of the core 33, depending on specifications of anoptical communication apparatus implemented with the optical fiber thusprocessed, more specifically on a design of a position detection systememployed in the apparatus. In such a case, the thin film of the metalmaterial m may be provided only on a portion of the clad 32 around thecore 33 as shown in FIG. 5, so as to constitute an optical fiber 3A′including a ring-shaped mirror surface region surrounding the core 33.

Also, in the first embodiment, a high-reflectance metal material m suchas Cr is vapor-deposited so as to form the level gap, thus creating avariation in reflectance by regions on the first facet 31. However, thematerial to be coated in a thin film is not limited to a metal materialsuch as Cr. Any other material may be employed, as long as the materialgenerates a difference in reflectance between the core 33 and clad 32required for clearly detecting the boundary therebetween based on thelight reflected by the first facet 31. For example, a material having areflectance at least higher than that of the core can make the surfaceof the clad 32 smooth, and thereby achieve a similar effect as the firstembodiment mentioned above. Even when employing a material other thanthe metal material m, the optical fiber 3A is subjected to similarprocessing to those shown in FIGS. 2A to 2F, and hence detailedexplanation of the method of processing in that case is not be repeatedherein.

Second Embodiment

FIG. 6 is a perspective view showing an optical fiber 3B processed by amethod of processing according to a second embodiment. In thisembodiment, to elements which are the same as those of the firstembodiment, the same reference numbers are assigned, and explanationsthereof will not be repeated. As shown in FIG. 6, the optical fiber 3Bis constituted such that on the first facet 31 the core 33 protrudes bya predetermined amount along an optical axis of the optical fiber 3B,and a top portion of the protruding core 33 is generally parallel to thefacet of the clad 32. The predetermined amount is set to be smaller thanλ/4, so as to cause diffraction when light is incident on both of theclad 32 and the protruding core 33. Here, λ stands for a wavelength ofthe incident light. In this embodiment, the predetermined amount is setat λ/8, so as to attain a largest difference in light intensity betweenthe light incident upon the core 33 and reflected thereby, and the lightincident upon the clad 32 and reflected thereby.

Implementing the optical fiber 3B, provided with the level gap thusformed, in an optical communication module such that the first facet 31serves as the light receiving facet for light from the laser diodeallows the optical communication module to detect the light intensitydistribution of the reflected light from the core 33 and the reflectedlight from the clad 32. Further, based on the detected light intensitydistribution, the optical communication module can precisely adjust theincident position of the light from the laser diode to the center of thecore 33, on the light receiving facet (first facet 31).

It should be noted that such position detection based on the lightintensity distribution on the light receiving facet cannot be performedby an optical communication module provided with a conventional opticalfiber. Also, the optical fiber according to the foregoing documents 1and 2, having a lens integrally formed therein, has a convex lens-shapedcore on a processed facet (an emitting facet). That is, a portion closeto the core is formed to be a convex lens shape. Accordingly, eventhough such an optical fiber is implemented in the optical communicationmodule such that the processed facet serves as the light receivingfacet, the foregoing diffraction cannot be effectively obtained, andtherefore the high-precision position detection and the positioningoperation based on the light intensity distribution cannot be performed.

To be more specific, the method of manufacturing according to thedocument 1 includes utilizing a difference in dissolving rate betweenthe clad and the core, by which the core, which is not supposed to bedissolved, is also dissolved. Therefore the optical fiber therebyfabricated can only provide insufficient optical transmissionefficiency, and is hence unsuitable for typical optical communication.The method of manufacturing according to the document 2 includescomplicated steps, which not only incurs an increase in manufacturingcost, but also fails to achieve a satisfactory yield. Such drawbacksincidental to the conventional technique can be eliminated by the methodof processing according to the second embodiment, through the followingsteps.

FIGS. 7A to 7F are schematic side views for explaining the method ofprocessing an optical fiber according to the second embodiment. Theresist r employed in the second embodiment is also of the negative type,as in the first embodiment. The method of processing according to thesecond embodiment is similar to the method of processing in the firstembodiment up to the exposing/developing process via the coating processand fixing process. Accordingly, the optical fiber 3B shown in FIG. 7Ais the same as FIG. 1, and the states thereof shown in FIGS. 7A to 7Dare the same as those shown in FIGS. 2A to 2D, respectively.

Referring to FIG. 7D, the optical fiber 3B that has undergone theexposing/developing process and carrying the residual resist only in aregion corresponding to the core 33 is subjected to an etching appliedto the clad 32 where the resist r is no longer present (an etchingprocess). The etching process is generally classified into a wet etchingand a dry etching, and either may be employed in the method ofprocessing according to the present invention. In this embodiment thedry etching is adopted, in order to form the level gap between the core33 and the clad 32 with high precision, since the accuracy in thisaspect is essential for detecting the light intensity distributionnecessary for high-precision position detection of light. For the dryetching process according to this embodiment, a FAB (fast atomic beam)processor may be suitably employed, because of its excellent anisotropicetching performance. FIG. 7E depicts the optical fiber 3B that hasundergone the etching process such that the height of the level gapbetween the core 33 and the clad 32 becomes λ/8. FIG. 7F depicts theoptical fiber 3B from which the resist r has been stripped. At thisstage, the optical fiber 3B obtains the structure shown in FIG. 6.

The foregoing is the method of processing an optical fiber according tothe second embodiment. Although the negative resist is employed in thesecond embodiment, employing a positive resist allows achieving anoptical fiber 3C which provides the same advantage as the optical fiber3B.

Third Embodiment

FIG. 8 is a perspective view showing an optical fiber 3C processed by amethod of processing according to a third embodiment. In thisembodiment, to elements which are the same as those of the first andsecond embodiments, the same reference numbers are assigned, andexplanations thereof will not be repeated. As shown in FIG. 8, in theoptical fiber 3C, the core 33 is recessed by a predetermined amount onthe first facet 31 along an optical axis of the optical fiber 3C, and abottom portion of the recessed core 33 and the facet of the clad 32 aregenerally parallel. The predetermined amount is preferably set at avalue smaller than λ/4. In this embodiment, the predetermined amount isset at λ/8 for the optical fiber 3C.

FIGS. 9A to 9F are schematic side views for explaining the method ofprocessing the optical fiber 3C according to the third embodiment. Themethod of processing according to the third embodiment is similar to themethod of processing in the second embodiment up to theexposing/developing process via the coating process and fixing process,except that a positive resist r is employed in the resist coatingprocess. Accordingly, the optical fiber 3C shown in FIG. 9A is the sameas FIG. 1, and the states thereof shown in FIGS. 9A to 9C are the sameas those of the optical fiber 3B shown in FIGS. 7A to 7C, respectively.

FIG. 9D depicts the optical fiber 3C that has just undergone theexposing/developing process. As already stated, a positive resist r isemployed in the third embodiment. Accordingly in the optical fiber 3Cshown in FIG. 9D, unlike the case of the optical fibers 3A and 3B, onlythe resist r applied to the exposed region corresponding to the core 33has been removed.

FIG. 9E depicts the optical fiber 3C subjected to an etching processafter the state of FIG. 9D. In the third embodiment also, the dryetching is performed as in the second embodiment. In the optical fiber3C shown in FIG. 9E, the core 33 has been etched until a level gapbetween the facet of the core 33 and the clad 32 becomes λ/8. FIG. 9Fdepicts the optical fiber 3C from which the resist r has been strippedafter the etching process, thus to expose the clad 32. At this stage,the optical fiber 3C obtains the structure shown in FIG. 8.

Fourth Embodiment

In the method of processing according to the second and the thirdembodiments, the level gap between the core 33 and the clad 32 on thefirst facet 31 is formed through the etching process. However, the levelgap can be formed by a different technique from the etching process.FIGS. 10A to 10F are schematic side views for explaining a method ofprocessing an optical fiber 3D according to a fourth embodiment. Themethod of processing according to the fourth embodiment is generallysimilar to the method of processing in the third embodiment up to theexposing/developing process. Accordingly, the optical fiber 3D shown inFIG. 10A is the same as FIG. 1, and the states thereof shown in FIGS.10A to 10C are the same as those of the optical fiber 3C shown in FIGS.9A to 9C, respectively.

In the method of processing according to the fourth embodiment, apredetermined material g is filled in a region corresponding to the core33 of the optical fiber under the state shown in FIG. 10D, so as to formthe level gap (FIG. 10E). As the material g, for example a glass (SiO₂)having generally the same refractive index as that of the core 33 may besuitably employed, so as not to impede the optical transmission. Thepredetermined material g is filled so as to accord with the height ofthe level gap, i.e. to achieve a thickness corresponding to λ/8. Then asshown in FIG. 10F, the resist r is stripped (lifted off), so that theoptical fiber 3D of generally the same structure as the optical fiber 3Bshown in FIG. 6 can be obtained.

Since the positive resist r is employed in the fourth embodiment, thematerial g is filled in the region corresponding to the core 33, so asto form the level gap. Such method of the fourth embodiment can bemodified by employing the negative resist instead. To achieve suchmodifications, a material identical to the clad 32, or a material havinggenerally the same refractive index as the clad 32 is coated on a regionwhere the resist has been removed, i.e. the region corresponding to theclad 32, so as to achieve the height of λ/8.

As described above, according to the embodiment, since the optical fiberis bent in a predetermined curvature radius, a portion of the lightincident upon the clad, out of the light passing through the opticalfiber from the second facet, is attenuated a plurality of times andhence can barely affect the exposure, even though such portion of thelight reaches the first facet 31. In contrast, the light that hasentered the core repeats total reflection inside the core, to be therebyintroduced only to the resist applied to the core in the first facet 31.As a result, the method (process) according to the embodiment allowsexposing only the resist applied to the core 33, with high precision.

Implementing the optical fibers 3A to 3D processed as above in theoptical communication module described here below allows the opticalcommunication module to constantly perform the positioning operation ofadjusting the incident position of the light form the laser diode to thecenter of the core 33 on the first facet 31.

Hereunder, the positioning operation performed by an opticalcommunication module 10 with respect to signal light for transmissionincident upon the light receiving facet 31 of the optical fiber 3A willbe described. FIG. 11 is a schematic diagram showing a configuration ofan optical communication module including the optical fiber 3A. Theoptical communication module 10 serves as an ONU that introduces theoptical fiber communication into a subscriber's house. The opticalcommunication module 10 supports interactive WDM communication utilizingan optical fiber for transmitting an upstream signal having a wavelengthof for example 1.3 μm, and for receiving a downstream signal having awavelength of for example 1.5 μm.

The optical communication module 10 shown in FIG. 11 includes a laserdiode LD, a first condenser lens 2, the optical fiber 3A, a secondcondenser lens 4, a photo detector 5, a controller 6 and an actuator 7.

The laser diode LD working as a light source of the transmission signallight is a surface emitting laser, which is capable of modulating thetransmission signal light according to the information to betransmitted. The laser diode LD, the first condenser lens 2 and theoptical fiber 3A are disposed such that the respective optical axescoincide with respect to each other. The optical fiber 3A is installedsuch that the first facet 31 confronts the first condenser lens 2. Inother words, the first facet 31 serves as a plane to which the lightfrom the laser diode LD is introduced. The transmission light having awavelength of 1.3 μm emitted by the laser diode LD is converged by thefirst condenser lens 2 toward the light receiving facet (first facet) 31of the optical fiber 3. The transmission light is transmitted to anoptical communication module (not shown) on a receiving side, via theoptical fiber 3A. This is how the optical communication is performed.

When performing a positioning operation with the optical communicationmodule 10, the light reflected by the light receiving facet 31 is led tothe second condenser lens 4. Here, processing the optical fiber facet 31so as to be inclined with respect to the optical axis eliminates theneed to provide a means of directing the reflected light to the secondcondenser lens 4, thereby achieving a reduction in the number ofcomponents. The second condenser lens 4 collects the reflected light anddirects the reflected light to the photo detector 5. The photo detector5 is located so as to be optically conjugate with the light receivingfacet 31. In other words, the light reflected at the center of theoptical fiber is incident generally upon the center of a receivingsurface of the photo detector 5.

The photo detector 5 is a quadrant photo-diode having four areas equallydivided by two boundary lines orthogonal to each other, intersecting atthe center of the receiving surface of the photo detector 5. The photodetector 5 transmits light amount data representing the light amount ineach area, to the controller 6.

As described above, the reflectance of the core 33 is far lower than theclad 32 (mirror surface). Accordingly, since the light reflected by thecore 33 is extremely faint, the photo detector 5 may fail to detect thereflected light. The photo detector 5 according to this embodiment is,therefore, provided with a higher sensitivity in the area where thelight reflected by the core 33 is to be introduced, so as to preciselydetect the amount of the light reflected by the core 33, on thereceiving surface of the photo detector 5.

The controller 6 performs, upon receipt of the light amount data of therespective areas of the photo detector 5, a negative feedback control sothat the light from the laser diode LD is incident upon the center ofthe core 33. More specifically, the controller 6 drives the firstcondenser lens 2 via the actuator 7 so as to move the incident positionof the light from the light source on the light receiving facet 31,until the amounts of light incident upon the respective areas becomeequivalent to each other. When the amounts of light incident upon theareas are equivalent to each other, the light from the laser diode LD isincident upon the center of the core 33.

The foregoing positioning operation is performed not only at an initialsetting in the manufacturing process of the optical communication module10, but is constantly carried out after turning on the opticalcommunication module 10 and during the execution of the opticalcommunication. In other words, the photo detector 5 constantly receivesthe light from the LD while the optical communication is being executed.Therefore, the controller 6 can perform the negative feedback controlsuch that the amounts of the light incident upon the areas becomeequivalent to each other, based on the light amount data constantly orperiodically transmitted from the photo detector 5.

That is how the optical communication module 10 implemented with theoptical fiber 3A performs the positioning operation. The opticalcommunication module 10 can equally perform the foregoing operation whenone of the optical fibers 3B to 3D is implemented in place of theoptical fiber 3A. When any of the optical fibers 3B to 3D is employed,however, the controller does not perform the negative feedback controlsuch that the amounts of the light incident upon the areas on thereceiving surface become equivalent to each other. Instead, thecontroller 6 performs the negative feedback control such that the lightintensity distribution of the light reflected by the light receivingfacet 31 accords with a predetermined distribution obtained when thelight from the LD is incident upon the center of the core 33.

According to the foregoing embodiments, the coating process, the fixingprocess and the resist coating process are performed in this sequence.However, in the method of processing according to the present invention,the coating, fixing and resist coating processes do not necessarily haveto be performed in this sequence. For example, the coating process andthe resist coating process may be first performed, before performing thefixing process.

Also, in the foregoing embodiments, the optical fiber is bent at oneposition in the fixing process, as shown in FIG. 1. As long as thecurvature radius is set within the desirable range, the optical fibermay be bent at a plurality of positions in the fixing process. Bendingthe optical fiber a plurality of times naturally increases the times(i.e. the number of points P) that the light incident upon the clad onthe second facet 34 (such as the light beams L2 and L3) hits theinterface between the UV-curing resin film 35 and the nylon 36, thuscausing the attenuation more frequently than in the embodiments. As aresult, the exposure process that achieves even more precise distinctionof the core and the clad on the first facet 31 can be performed.

Also, while the UV-curing resin film 35 is provided outside the clad inthe foregoing embodiment, for increasing the strength of the opticalfiber, UV-curing resin film 35 is not necessarily required if theoptical fiber has a sufficient strength. In this case, the nylon 36 iscoated over the clad. Therefore, finishing the outer surface of the cladin a rough surface enhances the light attenuation effect.

Further, according to the present invention, simply bending the opticalfiber, without providing the nylon coating, can provide the attenuationeffect with respect to the light beams L2 and L3 (see FIG. 3).Therefore, one of the methods according to the above mentionedembodiments may be adopted while taking into consideration the balancebetween the time and cost required for the processing of the opticalfiber and the expected precision level for the identification of theboundary between the core and the clad.

Further, in the foregoing embodiments the resist r is applied to agenerally entire region of the first facet 31. However, when a negativeresist is employed, the level gap can be duly formed provided that theresist is applied to a region at least including the entirety of thecore 33 on the first facet.

1. A method of processing an optical fiber having a core and a clad, theoptical fiber having a first facet and a second facet, the methodcomprising: fixing the optical fiber in a state in which at least aportion thereof is bent in a predetermined curvature radius; applying aresist to a region on the first facet at least including an entirety ofthe core; irradiating the second facet with light of a predeterminedwavelength so that only the resist applied to the core in the firstfacet is exposed to the light through an inside of the optical fiber;developing the resist; and forming a level gap at a boundary between thecore and the clad in the first facet utilizing the resist remainingafter the irradiating and the developing.
 2. The method according toclaim 1, further comprising finishing at least a portion of an outersurface of the clad in a rough surface.
 3. The method according to claim1, further comprising providing at least one coating on an outer surfaceof the clad.
 4. The method according to claim 1, further comprising:providing two or more coatings on an outer surface of the clad; andfinishing at least one of interfaces between the coatings in a roughsurface.
 5. The method according to claim 1, further comprisingproviding two or more coatings on an outer surface of the clad, whereinUV light is used as the light of the predetermined wavelength in theirradiating, and wherein at least one of the two or more coatings isformed of a UV-absorbing material.
 6. The method according to claim 5,wherein a coating which is one of the two or more coatings and is formedof the UV-absorbing material has a higher refractive index for the UVlight than an inner adjacent coating of the coating formed of theUV-absorbing material.
 7. The method according to claim 5, wherein nylonwhich is opaque to the UV light is used as the UV-absorbing material. 8.The method according to claim 1, wherein a following condition issatisfied:20≦R≦200  (1) where R (mm) represents the predetermined curvatureradius.
 9. The method according to claim 1, wherein the resist isapplied to an entire region of the first facet in the applying.
 10. Themethod according to claim 1, wherein the resist is a negative resist.11. The method according to claim 1, wherein the resist is a positiveresist.
 12. The method according to claim 10, wherein the forming of thelevel gap includes: performing a surface treatment at least within aregion close to the core on the clad in the first facet so as togenerate a difference in reflectance between the region and the core;and stripping the resist remaining on the first facet subjected to thesurface treatment.
 13. The method according to claim 12, wherein thesurface treatment is a treatment of vapor-depositing a metal material ina form of a thin film at least within the region close to the core onthe clad in the first facet.
 14. The method according to claim 1,wherein the forming of the level gap includes: performing an etching ona region where the resist is no longer present on the first facet; andstripping the resist remaining on the first facet, after the etching.15. The method according to claim 11, wherein the forming of the levelgap includes: filling a region where the resist is no longer present inthe first facet with a material that has generally the same refractiveindex as the optical fiber; and stripping the resist remaining on thefirst facet after the filling.